1. Field of the Invention
The present invention relates to a differential motion detection method using background image, and more particularly a motion detection method for detecting a moving object in an image utilizing digital image processing in order to control the operation of an image processing system.
2. Description of the Related Art
Techniques for detecting a moving object (travelling object) in the field of monitoring may be roughly classified into an inter-frame differential method which calculates a differential between frames, a corresponding point calculation method which calculates corresponding points in respective image portions of sequential images, a background differential method which calculates a differential between a background image and an input image, and so on.
Although the inter-frame differential method is simple in structure since it only calculates a differential between two images, a region obtained as a result of the differential calculation does not represent a target object itself, so that it is difficult to detect an image of a moving object, i.e., the target object only from the result of the differential calculation.
The corresponding point calculation method divides an image into small regions, calculates shape parameters characterizing the respective small regions, and correlates the shape parameters between images to calculate corresponding points in these images. Thus, the processing required by this method is so heavy that the detection of corresponding points encounters difficulties when an image includes a rotating object or an object which transforms its shape, since the shape parameters for the respective small regions frequently vary.
The background differential method assumes that an input image includes an image portion of an object (moving object) to be detected and a background image portion, and obtains the image of the object to be detected by subtracting the background image portion from the input image. This method provides favorable detection of a target object if a correct background image is defined. For this purpose, it can be thought to previously capture a background image without a target object. However, since a change in the background image will result in incorrect detection, a method of producing a background image portion plays an important role in the background differential method.
JP-A-62-114064 shows an approach for setting uniform integration parameters to a region subjected to motion detection processing in an image for the current background differential method. This approach represents an updated background image by the following equation (1): EQU B(n)=A.multidot.I(n)+(1-A).multidot.B(n-1) (1)
wherein I(n) is an input image, B(n-1) is a previous background image, and A is called a background integration parameter. The background integration parameter is a constant and common to all pixels. This approach, however, has a problem that a hole, a tail, and so on may be produced in a detected region to cause deformation of the detected region depending on a moving speed of an object to be detected, reflectivity of the surface of the object, and a used threshold value.
Background image updating methods include a method which employs a differential/absolute value image D(i, j) between an input image I(i, j) and a background image B(i, j), sets "0" to all the integration parameters A(i, j) in the equation (1), and replaces the background image with the input image (JP-A-1-7174), and a method which uses the reciprocals of absolute values of differential values as the integration parameters A(i, j). Either of the methods replaces the background image with the input image when an absolute value of a differential value is smaller than a threshold value, or smaller integration parameters are set to increase the degree of the replacement.
In the above-mentioned conventional techniques, however, when a moving object having a small absolute value of a differential value enters an input image, a background image is replaced with the input image, whereby the region of the moving object is included in the background image as noise. In view of updating the background image, it is desirable that the background image is not replaced with the region of the moving object.
Further, although the background differential method is said to be resistant to fluctuations in a background image, this is only in the case of slow fluctuations in an input image caused by a changing magnitude of sunlight, automatic gain control (AGC) of a camera, changes in aperture, and so on. However, when a sudden change occurs in an input image due to lightning, switching on or off of illumination, and so on, erroneous detection may result depending on a threshold value set for binary coding and to the values of the background integration parameters A(i, j).